STRUCTURAL CONSEQUENCES OF ELECTRON-TRANSFER REACTIONS .22. ELECTROCHEMICALLY INDUCED INSERTION OF RUTHENIUM ATOMS INTO A C-C BOND - REVERSIBLE SLICING OF A CYCLOOCTATETRAENE LIGAND

Oxidation of the pseudo-triple-decker complex Cp2Ru2(μ-cyclo-C8H8) (1) proceeds by a two-electron process at ambient temperatures to give the stable dication Cp2Ru2(μ-cat-C8H8)2+ (2). Whereas the bridging C8H8 moiety of 1 is a cyclooctatetraene ring, the CgHg group in 2 is an open chain, so that the dication is a “flyover” complex. Insertion of both metal atoms into a C-C bond of the cyclooctatetraene is thus observed as a consequence of the redox process. The flyover dication is efficiently reduced back to the original complex 1. Thus, 1 and 2 form a chemically reversible couple in which two-electron transfer leads to insertion and deinsertion of two metals into a C-C bond. The overall process is similar to those suggested regarding the metal-catalyzed cyclooligomerization of alkynes. The mechanism of the reaction has been studied by cyclic voltammetry, bulk coulometry, and other electrochemical methods. At low temperatures the two-electron oxidation wave of 1 becomes a one-electron wave owing to the stability of 1+. At higher temperatures 1+ isomerizes to the flyover monocation, which is then oxidized to 2. The lifetime of 1+ is ca. 50 ms at 263 K. The overall mechanism for the reaction 1 ⥨ 2 + 2e- appears to follow an ECE-EEC route. 1H and 13C NMR spectra of 2 were analyzed in terms of the flyover structure. Complex 1 is fluxional and was found to exhibit a rearrangement mechanism of clockwise 1,3-metal shifts of one of the Ru atoms on the cyclooctatetraene ring. This is identical with that observed earlier for the isoelectronic Cp2Rh2(μ-C8H8)2+, but the activation barrier is higher by ca. 1.4 kcal/mol at 320 K. The X-ray crystallographic structures of 1 and 2 have been determined: 1, Cp2Ru2(μ-cyclo-C8H8), monoclinic, C2/c, a = 13.880 (4) Å, b = 6.289 (1), c = 16.514 (4) Å, β = 95.50 (2)°, V = 1435.0 (7) Å3, Z = 4, R(F) = 2.51%; for 2, [Cp2Ru2(μ-cat-C8H8)][PF6]20.5C6H6, triclinic, Pi, a = 9.237 (3) Å, b = 9.234 (3) Å, c = 16.151 (5) Å, α = 80.31 (3)°, β = 74.09 (2)°, γ = 68.09 (2)°, V= 1225.8 (7) Å3, Z = 2, R(F) = 3.89%. © 1990, American Chemical Society. All rights reserved.